An accessory drive belt for an engine is driven by the crankshaft and may be operatively connected by pulleys to an air pump, an air conditioning compressor, a water pump, a power steering pump, and an alternator for driving these devices. The pulley associated with the alternator is much smaller than the pulley associated with the crankshaft, so the alternator rotor rotates at a relatively high speed. Also, the alternator rotor has a relatively high mass, which results in a high rotational inertia due to the high speed.
The speed profile of the crankshaft may vary significantly during engine cycles, particularly when the engine has a small number of active cylinders. Accordingly, due to the rotational inertia of the alternator rotor, significant positive or negative tension may occur in the belt between the crankshaft and the alternator as speed variations occur in the crankshaft. As a result of these tension variations in the belt, belt noise or belt chirp may occur. Other problems include vibration and reduced durability of the drive belt system.
When engine cylinders are deactivated, this belt noise problem may be exacerbated as the rotational inertia of the alternator rotor reacts to greater changes in crankshaft speed. Because certain engine cylinders have been deactivated, the time lapse between changes in acceleration and deceleration of the crankshaft result in greater amplitude of velocity changes in the crankshaft, which can cause significant changes in tension in the drive belt as inertia in the alternator rotor is overcome.
These belt noise problems may also occur in drivetrain systems having a high overdrive ratio, or in diesel engines.